Method and system for producing a tyre tread with lugs

ABSTRACT

A method of producing a tyre tread with lugs; the tread having a tread base, and a number of lugs projecting upwards from the tread base; and the method including the steps of: preparing a green-rubber blank tread thinner at the sides and thicker at the centre to define a raised centre portion; applying green-rubber blocks to the areas of the blank tread where the lugs are to be formed, which blocks rest centrally against the raised centre portion; and curing the blank tread, together with the blocks, in a curing mold negatively reproducing the pattern of the tread.

TECHNICAL FIELD

The present invention relates to a method and system for producing atyre tread with lugs.

The present invention may be used to advantage for producing farmvehicle tyre treads. It is important to note that the term ‘farm vehicletyre’ is intended to mean a tyre used on off-road work vehicles, and sorefers not only to farm vehicles but also earthmoving machinery orsimilar.

BACKGROUND ART

A farm vehicle tyre has a tread with a toroidal base surface thatextends about a central axis of rotation; and a number of lugs projectradially from the base surface, and extend substantially crosswise tothe travelling direction of the tyre.

At present, to produce a farm vehicle tyre, a tyre casing is preparedand wound onto a building drum; the tread belts are wound about thecasing on the building drum, and a smooth, green-rubber blank tread iswound over the belts; and the casing, wound with the belts and the blanktread, is inserted inside a curing mold negatively reproducing the treadpattern. During the curing process, the rubber of the blank tread isheated and subjected to high pressure to adapt to the shape of thecuring mold and so form the tread pattern.

The tread pattern includes the lugs, which are exceptionally large and,above all, project upwards considerably from the rest of the tread (evenby as much as ten centimetres at the highest point). Forming the lugs inthe curing mold therefore involves considerable rubber displacementinside the mold, i.e. a large amount of rubber must migrate from onepart of the mold to another. This rubber displacement inside the curingmold, i.e. the large amount of rubber that has to be pressured intoforming the lugs, prevents the formation of an evenly thick tread base.In other words, migration of a large amount of rubber inside the curingmold results in the formation of an ‘uneven’ tread base (i.e. differingwidely in thickness) which may result in defects, rippling and unsightlyflaws.

To prevent too thin a rubber thickness at the thinnest parts of thetread base, the average thickness of the tread base must be ‘oversized’with respect to an optimum thickness that could be used if the treadbase were of perfectly even thickness. In other words, to prevent toothin a rubber thickness at the thinnest parts of the tread base, thewhole tread base must be made thicker using more rubber. Oversizing theaverage thickness of the tread base means more material (rubber) isrequired to produce the tyre (with no advantage whatsoever in terms ofnominal tyre performance), thus increasing the manufacturing cost andweight of the tyre.

Moreover, because the size of the green-rubber tread cross section iscalculated according to tyre size, the known production method describedabove requires a different green-rubber tread cross section for eachtyre radius. In other words, the green-rubber tread cross section of a420/85 R24 tyre, for example, cannot be used for a 420/85 R28, R30, R34or R38 tyre.

To eliminate these drawbacks, it has been proposed to reduce thethickness of the blank tread wound about the casing (with the belts inbetween), and to apply green-rubber blocks to the areas of the blanktread where the lugs are to be formed. This reduces the amount of rubberthat has to migrate from one part of the curing mold to another, byproviding more rubber (in the form of rubber blocks) where it is needed(to form the lugs) right from the outset. Even using rubber blocks,however, the thickness of the tread base still remains significantlyuneven, especially at the shoulders of the tyre where the lugs arelarger. In other words, even using rubber blocks, the average thicknessof the tread base must be ‘oversized’ with respect to an optimumthickness that could be used if the tread base were of perfectly eventhickness.

Patent Application WO2012001544A1 describes a method of producing alugged tyre, whereby a green-rubber blank tread is produced; rubberblocks are applied to the areas of the blank tread where the lugs are tobe formed; and the blank tread, together with the blocks, is cured in acuring mold.

DESCRIPTION OF THE INVENTION

It is an object of the present invention to provide a method and systemfor producing a tyre tread with lugs, which method and system aredesigned to eliminate the above drawbacks and, in particular, are cheapand easy to implement.

According to the present invention, there are provided a method andsystem for producing a tyre tread with lugs, as claimed in theaccompanying Claims.

BRIEF DESCRIPTION OF THE DRAWINGS

A number of non-limiting embodiments of the present invention will bedescribed by way of example with reference to the attached drawings, inwhich:

FIG. 1 shows a view in perspective of a farm vehicle tyre;

FIG. 2 shows a front view of the FIG. 1 farm vehicle tyre;

FIG. 3 shows a side view of the FIG. 1 farm vehicle tyre;

FIG. 4 shows a schematic cross section of part of the FIG. 1 farmvehicle tyre;

FIG. 5 shows a schematic of a system, in accordance with the presentinvention, for producing the FIG. 1 farm vehicle tyre;

FIG. 6 shows a schematic of a building drum of the FIG. 5 system;

FIG. 7 shows a plan view of a portion of a blank tread used in the FIG.5 system;

FIG. 8 shows a cross section along line VIII-VIII of the FIG. 7 blanktread;

FIG. 9 shows a schematic view in perspective of an extruder forextruding the FIG. 7 blank tread;

FIG. 10 shows a schematic view in perspective of an extruder forextruding green-rubber blocks for application to the FIG. 7 blank tread;

FIG. 11 shows a schematic plan view of a crosswise cut to separate twoconsecutive FIG. 10 blocks;

FIG. 12 shows a plan view of a portion of an alternative embodiment of ablank tread used in the FIG. 5 system;

FIG. 13 shows a cross section along line XIII-XIII of the FIG. 12 blanktread;

FIG. 14 shows a schematic view in perspective of an extruder forextruding green-rubber blocks for application to the FIG. 12 blanktread;

FIG. 15 shows a schematic plan view of a crosswise cut to shape a FIG.14 block;

FIG. 16 shows a plan view of a portion of a further embodiment of ablank tread used in the FIG. 5 system;

FIG. 17 shows a cross section along line XVII-XVII of the FIG. 16 blanktread;

FIG. 18 shows a schematic of the FIG. 6 building drum used in analternative embodiment;

FIG. 19 shows a view in perspective of a different embodiment of a farmvehicle tyre;

FIG. 20 shows a front view of the FIG. 19 farm vehicle tyre.

PREFERRED EMBODIMENTS OF THE INVENTION

Number 1 in FIGS. 1-4 indicates as a whole a farm vehicle tyre.

Tyre 1 comprises a toroidal casing 2, which extends about a central axisof rotation and supports tread belts 3 wound about casing 2, and a tread4 wound about casing 2 and over belts 3. Tread 4 comprises a tread base5, which completely covers casing 2 and is of substantially constantthickness; and a number of lugs 6, which project upwards (i.e. extendradially outwards) from tread base 5 and are arranged symmetricallyabout the axis of rotation. Each lug 6 curves from the centreline oftyre 1 to a corresponding shoulder of tyre 1, has atruncated-cone-shaped cross section (tapering away from tread base 5),and has a front wall and a rear wall with respect to a given rollingdirection 7 of tyre 1. Each lug 6 has a lateral portion 8 at a shoulderof tyre 1; and a centre portion 9 located in the centre area of tyre 1.As shown clearly in FIG. 4, each lug 6 is thinner at centre portion 9,and thicker at lateral portion 8. As shown clearly in FIG. 2, centreportion 9 of each lug 6 has a greater curvature than lateral portion 8(roughly speaking, lateral portion 8 may be assumed to be substantiallystraight).

Number 10 in FIG. 5 indicates as a whole a production system forproducing farm vehicle tyre 1. Production system 1 comprises a buildingunit 11 for producing casing 2 of tyre 1; a winding unit 12, which windsfirst belts 3 and then a green-rubber blank tread 13 about casing 2; anapplication unit 14, which applies green-rubber blocks 15 to the areasof blank tread 13 where lugs 6 are to be formed; and a curing unit 16for curing casing 2, together with belts 3, blank tread 13 and blocks15, in a curing mold 17 negatively reproducing the pattern of tread 4.In an alternative embodiment, blocks 15 may be made of cured, as opposedto green, rubber, i.e. may be pre-cured.

As shown in FIG. 6, casing 2 produced on building unit 11 is mountedonto a building drum 18 powered to successively receive belts 3, blanktread 13 and blocks 15. In other words, once casing 2 is mounted onbuilding drum 18, belts 3 are wound about casing 2, blank tread 13 iswound about casing 2 and over belts 3, and blocks 15 are applied toblank tread 13 wound about casing 2. Each block 15 has an outer wall 19at a shoulder of blank tread 13; and an inner wall 20 opposite outerwall 19 and located in the centre area of blank tread 13.

In the FIGS. 6, 7 and 8 embodiment, blank tread 13 is thinner at thesides and thicker at the centre, so as to define a raised centre portion21 against which inner walls 20 of blocks 15 rest. Inside curing mold17, blocks 15 therefore eventually form lateral portions 8 of lugs 6,and raised centre portion 21 eventually forms centre portions 9 of lugs6 (that is, the centre portion of the pattern of tread 4). In otherwords, inside curing mold 17, the rubber needed to form lateral portions8 of lugs 6 comes mostly from blocks 15, while the rubber needed to formcentre portions 9 of lugs 6 (which define the centre portion of thepattern of tread 4) comes mostly from raised centre portion 21 of blanktread 13.

In a preferred embodiment shown in FIG. 8, blank tread 13 is of constantthickness outwards of raised centre portion 21; and raised centreportion 21 of blank tread 13 has a trapezoidal cross section.(Alternatively, raised centre portion 21 of blank tread 13 may have arectangular cross section).

In a preferred embodiment shown in FIG. 9, blank tread 13 has a constantcross section along its whole length, and is extruded by an extruder 22.

In a preferred embodiment shown in FIG. 7, each block 15 is positionedat an angle with respect to raised centre portion 21, i.e. with respectto a circumferential plane of blank tread 13 (following, as stated, theareas where lugs 6 are to be formed) so that block 15 forms an acuteangle α with a circumferential plane.

To distribute the green rubber more evenly inside curing mold 17, theinner wall 20 of each block 15 contacting raised centre portion 21 isshaped on a slant to form an acute angle β with the longitudinal axis ofblock 15. Preferably (but not necessarily), acute angle β equals acuteangle α, so that inner wall 20 is parallel to raised centre portion 21.

Preferably, to distribute the green rubber more evenly inside curingmold 17, the inner wall 20 of each block 15 contacting raised centreportion 21 is shaped on a slant to also form an acute angle δ with thetangential plane of blank tread 13. Preferably (but not necessarily),acute angle δ is supplementary to the angle formed by the lateral wallsof raised centre portion 21 with the tangential plane of blank tread 13.In other words, inner wall 20 of each block 15 slopes doubly: by acuteangle β with respect to the longitudinal axis of block 15, and by acuteangle δ with respect to the tangential plane of blank tread 13.

Preferably, to distribute the green rubber more evenly inside curingmold 17, the outer wall 19 of each block 15 is also shaped on a slant toform an acute angle γ with the longitudinal axis of block 15. Preferably(but not necessarily), acute angle γ equals acute angle α, so that outerwall 19 is parallel to raised centre portion 21 and inner wall 20. Inother words, preferably (but not necessarily), the two opposite walls 19and 20 of each block 15 are parallel to each other and slope withrespect to the longitudinal axis of block 15.

Preferably, to distribute the green rubber more evenly inside curingmold 17, outer wall 19 of each block 15 is also shaped on a slant toform an acute angle ε with the tangential plane of blank tread 13.Preferably (but not necessarily), acute angle ε equals angle δ, so outerwall 19 is parallel to inner wall 20. In other words, outer wall 19 ofeach block 15 slopes doubly: by acute angle γ with respect to thelongitudinal axis of block 15, and by acute angle ε with respect to thetangential plane of blank tread 13.

In a preferred embodiment shown in FIG. 10, each block 15 has a constanttrapezoidal cross section along its whole length, and is extruded by anextruder 23. More specifically, extruder 23 extrudes a continuous rubberstrip with the same trapezoidal cross section as blocks 15; and blocks15 are cut off the continuous rubber strip from extruder 23 by acrosswise cut in a plane inclined at an angle equal to acute angle β(normally equal to acute angle γ), and at a further angle equal to acuteangle δ) normally equal to acute angle ε).

In the FIG. 7-10 embodiment, each block 15 has a constant trapezoidalcross section along its whole length. In the FIG. 12-14 embodiment, eachblock 15 has a trapezoidal cross section varying lengthwise, so as to bethinner at inner wall 20 and thicker at outer wall 19. Preferably, thethickness of each block 15 increases evenly (linearly) from inner wall20 to outer wall 19. Alternatively, the thickness of each block 15 mayincrease discontinuously (in steps) from inner wall 20 to outer wall 19.

Varying the cross section of each block 15 lengthwise provides forfurther improving rubber displacement inside curing mold 17, byproviding more rubber at the shoulders of tyre 1 (i.e. at lateralportions 8, where lugs 6 are thicker) and less rubber in the centre areaof tyre 1 (i.e. at centre portions 9, where lugs 6 are thinner). On theother hand, forming blocks 15 with a lengthwise-varying cross section ismore complicated (and therefore takes longer and is more expensive).

As shown in FIG. 14, blocks 15 may be formed by extruding, on extruder23, a continuous rubber strip with a cross section identical to thelongitudinal section of blocks 15; and by cutting blocks 15 off thecontinuous rubber strip by a crosswise cut in an inclined plane (asshown clearly in FIG. 14, the blocks are cut off the continuous rubberstrip so they face alternately upwards and downwards, by continuallyinverting the angle of the cutting plane). As shown in FIG. 15, once cutoff the continuous rubber strip, each block 15 is cut at opposite endsto form inner wall 20 and outer wall 19 sloping at corresponding(possibly different) angles β and γ, and at corresponding (possiblydifferent) angles δ and ε.

In a further embodiment shown in FIGS. 16 and 17, when (and only when)blocks 15 vary lengthwise in cross section (i.e. are thinner at innerwall 20 and thicker at outer wall 19), a blank tread 13 of constantthickness (i.e. with no raised centre portion 21) may be used.

In one possible embodiment, to improve grip of blocks 15 to blank tread13 (and so prevent accidental movement of blocks 15 when insertingcasing 2 inside curing mold 17), cement is applied between a bottomsurface of each block 15 and a top surface of blank tread 13.Alternatively or in addition to cement, to improve grip of blocks 15 toblank tread 13, the bottom surface of each block 15 and/or the topsurface of blank tread 13 may be pre-heated (e.g. using infrared-raylamps for non-contact heating).

In one possible embodiment, blank tread 13 is made from a softer firstrubber compound, and blocks 15 are made from a different second rubbercompound harder than the first. This embodiment makes it possible toimprove both performance and working life of tyre 1, by making lugs 6(substantially formed from the rubber of blocks 15) harder (andtherefore more wear resistant), while tread base 5 (substantially formedfrom the rubber of blank tread 13) is softer and so able to betterdistribute mechanical stress produced by the rolling movement of thetyre.

In the FIG. 6 embodiment, building unit 11 produces a new casing 2 forproducing a new tyre 1 (in fact, belts 3 are first wound about the‘virgin’ casing 2). In an alternative embodiment shown schematically inFIG. 18, building unit 11 starts with a worn tyre 1 for retreading.First of all, the worn tread is removed from tyre 1 to expose anintermediate surface of casing 2 underneath (at any rate, a surface overthe existing belts 3), and the intermediate surface of casing 2 isskived. Once the skiving operation is completed, casing 2 is wound witha green-rubber cushion 25; blank tread 13 is then wound about casing 2,on top of green-rubber cushion 25; and blocks 15 are applied asdescribed above to blank tread 13. Finally, casing 2, together withgreen-rubber cushion 25, blank tread 13 and blocks 15, is cured in atoroidal curing mold 17 to produce a retreaded tyre 1.

In the above embodiments, blocks 15 are applied to blank tread 13 afterit is wound about casing 2. In an alternative embodiment, blocks 15 maybe applied to the flat blank tread 13, which is then wound, completewith blocks 15, about casing 2. In this embodiment, the flat blank tread13, complete with blocks 15, may even be cured to form a pre-cured tread(PCT) 4, which is then wound about casing 2 as part of a ‘cold’ tyrebuilding or retreading process.

In the FIG. 1-4 embodiment, the tread 4 pattern only comprises lugs 6projecting upwards (i.e. extending radially outwards) from tread base 5.In the FIGS. 19 and 20 embodiment, the tread 4 pattern, in addition tolugs 6, also comprises a central rib 26, which projects upwards (i.e.extends radially outwards) from tread base 5, is annular in shape (i.e.is in the form of a ring seamlessly encircling the whole of tread 4),and is located centrally along the centreline of tread 4. In thisembodiment, centre portions 9 of lugs 6 blend (i.e. merge) seamlesslywith central rib 26. The FIGS. 19 and 20 tread 4 with central rib 26 ispreferably produced from blank tread 13 with raised centre portion 21,which minimizes the amount of rubber migration, inside curing mold 17,required to form central rib 26 of tread 4. Raised centre portion 21 ofblank tread 13 is normally designed to form both central rib 26 of tread4 and centre portions 9 of lugs 6. Alternatively, raised centre portion21 of blank tread 13 is designed to only form central rib 26 of tread 4,while blocks 15 form the whole of lugs 6 (i.e. both lateral portion 8and centre portion 9 of each lug 6).

The above method of producing tread 4 of farm vehicle tyre 1 hasnumerous advantages.

First and foremost, the method described provides for minimizing rubbermigration inside curing mold 17, and so achieving a highly even treadbase 5, i.e. of more or less constant thickness over its whole area. Assuch, no ‘oversizing’ of the mean thickness of tread base 5 isnecessary, thus enabling a significant reduction in the cost and totalweight of tyre 1 for a given performance. It is important to note that,for casing 2, complete with blank tread 13 and blocks 15, to fitsmoothly inside curing mold 17, there are strict geometric limitationsas to the size of blank tread 13, and especially the size and shape ofblocks 15. By virtue of raised centre portion 21 of blank tread 13and/or the variation in the cross section of blocks 15, it is possibleto achieve more or less perfect rubber distribution (i.e. to provide therubber exactly where it is needed in the tread 4 pattern), as well assmooth insertion of casing 2, complete with blank tread 13 and blocks15, inside curing mold 17.

Moreover, the method described allows the same size green-rubber treadcross section to be used for producing same-type tyres of differentradii (i.e. permits ‘size rim scaling’) by maintaining a constantthickness of blank tread 13. That is, the different amount of rubberrequired to form a different number of lugs 6 is obtained by varying thenumber of blocks 15, with no change whatsoever in the thickness of blanktread 13. For example, the same size green-rubber tread cross sectionmay be used to produce tyres 420/85 of radius R24, R28, R30, R34 andR38.

1. A method of producing a tyre tread with lugs; the tread comprising atread base; and a number of lugs, each of which projects upwards fromthe tread base, and has a lateral portion located at a shoulder of thetyre, and a centre portion located in the centre area of the tyre; themethod comprising the steps of: preparing a green-rubber blank tread;applying rubber blocks substantially to the areas of the blank treadwhere the lugs are to be formed, each block having an outer wall locatedat a shoulder of the blank tread, and an inner wall opposite the outerwall and located in the centre area of the blank tread; and curing theblank tread, together with the blocks, in a curing mold negativelyreproducing the pattern of the tread; wherein the blank tread is thinnerat the sides and thicker at the centre, so as to define a raised centreportion, against which the inner walls the blocks rest, so the blockseventually define at least the lateral portions of the lugs, and theraised centre portion eventually defines the centre portion of thepattern of the tread.
 2. A method as claimed in claim 1, wherein,outwards of the raised centre portion, the blank tread is of constantthickness.
 3. A method as claimed in claim 1, wherein the raised centreportion has a trapezoidal cross section.
 4. A method as claimed in claim1, and comprising the further steps of: positioning each block at anangle with respect to the raised centre portion, so the block forms afirst acute angle with a circumferential plane; and shaping the innerwall and/or the outer wall of each block at an angle, so the inner walland/or outer wall form a second acute angle and third acute anglerespectively with the longitudinal axis of the block.
 5. A method asclaimed in claim 4, and comprising the further step of shaping the innerwall and/or the outer wall of each block at an angle, so the inner walland/or outer wall form a fourth acute angle and a fifth acute anglerespectively with the tangential plane of the blank tread.
 6. A methodas claimed in claim 4, and comprising the further steps of: extruding acontinuous rubber strip having a cross section identical to the crosssection of the blocks; and cutting the blocks off the continuous rubberstrip by means of an inclined crosswise cut.
 7. A method as claimed inclaim 1, wherein each block has a constant cross section along its wholelength.
 8. A method as claimed in claim 1, wherein each block varieslengthwise in cross section, so it is thinner at the inner wall andthicker at the outer wall.
 9. A method a claimed in claim 8, whereineach block increases evenly in thickness from the inner wall to theouter wall.
 10. A method as claimed in claim 8, and comprising thefurther steps of: extruding a continuous rubber strip having a crosssection identical to the longitudinal section of the blocks; and cuttingthe blocks off the continuous rubber strip by means of an inclinedcrosswise cut.
 11. A method as claimed in claim 10, and comprising thefurther steps of: positioning each block at an angle with respect to theraised centre portion, so the block forms a first acute angle with acircumferential plane; and cutting the inner wall and/or the outer wallof each block on a slant, so the inner wall and/or outer wall form asecond acute angle and third acute angle respectively with thelongitudinal axis of the block.
 12. A method as claimed in claim 10, andcomprising the further step of cutting the inner wall and/or the outerwall of each block on a slant, so the inner wall and/or outer wall forma fourth acute angle and a fifth acute angle respectively with thetangential plane of the blank tread.
 13. A method as claimed in claim 1,wherein the blank tread has a constant cross section along its wholelength.
 14. A method as claimed in claim 1, wherein the step of applyingeach block comprises first applying cement between a bottom surface ofthe block and a top surface of the blank tread.
 15. A method as claimedin claim 1, wherein the step of applying each block comprises preheatinga bottom surface of the block and/or a top surface of the blank tread.16. A method as claimed in claim 1, and comprising the further steps of:making the blank tread from a first rubber compound; and making theblocks from a second rubber compound different from and harder than thefirst rubber compound.
 17. A method as claimed in claim 1, wherein: thetread comprises a central rib, which projects upwards from the treadbase, is annular in shape, and is located centrally along the centrelineof the tread; and the raised centre portion of the blank tread definesthe central rib of the tread.
 18. A method as claimed in claim 1, andcomprising the further steps of: preparing a new casing of green rubber;winding belts about the casing; winding the blank tread about thecasing, on top of the belts; applying the blocks to the blank treadwound about the casing; and curing the casing, together with the belts,the blank tread and the blocks, in the curing mold to completeproduction of a new tyre.
 19. A method as claimed in claim 1, andcomprising the further steps of: removing the worn tread from a usedtyre to expose an intermediate surface of the casing underneath; skivingthe intermediate surface of the casing; winding a green-rubber cushionabout the casing; winding the blank tread about the casing, on top ofthe green-rubber cushion; applying the blocks to the blank tread woundabout the casing; and curing the casing, together with the green-rubbercushion, the blank tread and the blocks, in the curing mold to completeproduction of a retreaded tyre.
 20. A system for producing a tyre treadwith lugs; the tread comprising a tread base; and a number of lugs, eachof which projects upwards from the tread base, and has a lateral portionlocated at a shoulder of the tyre, and a centre portion located in thecentre area of the tyre; the system comprising: an extruder for forminga green-rubber blank tread; a unit for applying rubber blockssubstantially to the areas of the blank tread where the lugs are to beformed, each block having an outer wall located at a shoulder of theblank tread, and an inner wall opposite the outer wall and located inthe centre area of the blank tread; and a curing unit for curing theblank tread, together with the blocks, in a curing mold negativelyreproducing the pattern of the tread; wherein the extruder is designedto make the blank tread thinner at the sides and thicker at the centre,so as to define a raised centre portion, against which the inner wallsof the blocks rest, so the blocks eventually define the lateral portionsof the lugs, and the raised centre portion eventually defines the centreportion of the pattern of the tread.